The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface, and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of feet or meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. In addition to this conventional drilling equipment, the system also relies on some sort of drilling fluid, in most cases a drilling “mud” which is pumped through the inside of the pipe, which cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevent hydrocarbon influx from the formation into the wellbore which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well away from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) a drill bit; 2) a steerable downhole mud motor of rotary steerable system; 3) sensors of survey equipment (Logging While Drilling (LWD) and/or Measurement-while-drilling (MWD)) to evaluate downhole conditions as well depth progresses; 4) equipment for telemetry of data to surface; and 5) other control mechanisms such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a metallic tubular.
As an example of a potential drilling activity, MWD equipment is used to provide downhole sensor and status information to surface in a near real-time mode while drilling. This information is used by the rig crew to make decisions about controlling and steering the well to optimize the drilling speed and trajectory based on numerous factors, including lease boundaries, locations of existing wells, formation properties, and hydrocarbon size and location. This can include making intentional deviations from an originally-planned wellbore path as necessary based on the information gathered from the downhole sensors during the drilling process. The ability to obtain real time data during MWD allows for a relatively more economical and more efficient drilling operation.
Known MWD tools contain essentially the same sensor package to survey the well bore but the data may be sent back to surface by various telemetry methods. Such telemetry methods include but are not limited to the use of hardwired drill pipe, acoustic telemetry, use of fibre optic cable, Mud Pulse (MP) telemetry and Electromagnetic (EM) telemetry. The sensors are usually located in an electronics probe or instrumentation assembly contained in a cylindrical cover or housing, located near the drill bit.
Mud Pulse telemetry involves creating pressure waves in the drill mud circulating inside the drill string. Mud is circulated from surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. The pressure pulses are achieved by changing the flow area and/or path of the drilling fluid as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling fluid. The pressure differentials or pulses may be either negative pulses or positive pulses. Valves that open and close a bypass stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. All negative pulsing valves need a high differential pressure below the valve to create a sufficient pressure drop when the valve is open, but this results in the negative valves being more prone to washing. With each actuation, the valve hits against the valve seat to ensure it completely closes the bypass; the impact can lead to mechanical and abrasive wear and failure. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Some valves are hydraulically powered to reduce the required actuation power typically resulting in a main valve indirectly operated by a pilot valve. The pilot valve closes a flow restriction which actuates the main valve to create a pressure drop. Pulse frequency is typically governed by pulse generating motor speed changes. The pulse generating motor requires electrical connectivity with the other elements of the MWD probe such as the battery stack and sensors.
In mud pulser systems, as well as in other downhole tools, the pulse generating motor driveline system is subjected to extreme pressure differentials of about 20,000 psi between the external and internal aspects of the tool. To accommodate this large pressure differential, the borehole drilling fluid is allowed access to areas of the tool which are positioned on one side of a compensation mechanism. Pressure is equalized on the other side of the pressure compensation mechanism within the tool using clean, non-drilling fluid such as hydraulic fluid or silicon oil. Various systems have been used to provide pressure compensation including metallic bellows, rubber compensation membranes, and piston compensations with springs. Given the large temperature differentials from surface to downhole, especially in colder drilling climates, there is a high chance of temperature related failures for MWD tool components, in particular rubber membranes used for pressure compensation.
A pressure compensating device is described in WO 2012/130936 which utilizes pistons and fluid to provide pressure compensation via a dual section chamber within a housing. The device allows fluid communication through borehole ports to prevent collapse or bulging of the compensation device resulting from thermal expansion of the hydraulic fluid contained in one of the sections of the chamber. A different pressure compensating device is described in WO 2010/138961, which includes a metal membrane that can compensate for large oil volumes. The metal is capable of elastic deformation and has a shape chosen to optimize such deformation in a desired manner to compensate for the temperature and pressure effects experienced in downhole conditions. U.S. Pat. No. 8,203,908 describes a mud pulser system in which the spline shaft is surrounded by lubricating fluid which is pressurized against the downhole hydrostatic pressure using a bellows style pressure compensator. In addition to the bellows seal, the system has a dual seal which maintains the integrity of the lubrication chamber during operation and during replacement of the bellows seal for maintenance.
During MP telemetry the operation of a mud pulser can cause wear and breakdown of a seal which fluidly seals the rotating driveshaft of the mud pulser from the external drilling mud. The motor of the mud pulser is typically enveloped in lubricating oil which is contained in the pulser housing by the seal. With time, oil tends to leak out and drilling mud tends to leak in through the worn seal. This requires replacement of the seal before any substantial amount of mud leaks in. Mud within the motor housing is detrimental to the operation of the motor, bearings and gearbox, and these components will typically be destroyed if a substantial amount of drilling mud enters the motor housing.
Though seals are relatively simple in design and are used extensively in tools for directional drilling, there are a variety of downhole effects related to the vibration, pressure differential and temperature shocks that can cause seal failure. The seals play a vital role in maintaining the integrity of the mud pulse devices. For example, in rotor/stator configurations that use a blade style rotor, there is a small gap between the rotor blades and the stator. Where the driveshaft exits the stator to connect with the rotor, a seal is typically positioned at the shaft gap to prevent drilling mud ingression into driveline components. The seal is subject to high degrees of abrasion due to turbulence of the mudflow within the small gap between the rotor and stator faces; as such the seal is prone to wear and failure. Failure of the seal leads to the driveline components coming in contact with the drilling fluid which is detrimental to operation.